AbstractThis study evaluates four satellite-estimated data sets of surface longwave radiation (LW), the Global Energy and Water Cycle Experiment-Surface Radiation Budget (GEWEX-SRB), the International Satellite Cloud Climatology Project-Flux Data (ISCCP-FD), the Clouds and Earth Radiant Energy System-Gridded Radiative Fluxes and Clouds (CERES-FSW), and a newly developed data set from the Moderate Resolution Imaging Spectroradiometer (MODIS), with ground measurements at 15 sites of 2003. GEWEX-SRB (1 degrees*degrees 1) and ISCCP-FD (280 km*280 km) irradiance are available at 3-hourly intervals, CERES-FSW (1 degrees*degrees 1) and MODIS (1 km*1 km) irradiance are available at satellite overpass time. Evaluation is carried out separately for clear-sky, cloudy sky and all-sky conditions. Results show that, under all-sky conditions, downwelling LW (LWDN) in ISCCP-FD is overestimated while in CERES-FSW it is underestimated; but for LWDN in GEWEX-SRB and upwelling LW (LWUP) in all data sets, estimation errors are region-dependent: no uniform trend can be found. Under clear-sky conditions, LWDN in most data sets is underestimated, with some exceptions; LWUP in GEWEX-SRB and ISCCP-FD is overestimated while in CERES-FSW and MODIS it is underestimated with some exceptions. Under cloudy sky conditions, LWDN is overestimated in GEWEX-SRB and ISCCP-FD while it is underestimated in CERES-FSW; LWUP in most data sets is underestimated, with some exceptions. All data sets perform better when estimating LWUP than LWDN, during the nighttime than during the daytime, and under clear-sky conditions than under cloudy sky conditions. However, there are substantial errors in some regions, such as LWDN overestimation by ISCCP-FD on the Qinghai-Tibetan Plateau. Such errors are related to the errors of input precipitable water vapor, temperature, and elevation differences.